Various systems are known in which an interrogation device (reader) can determine the respective identities of two or more transponders (tags) lying within its detection field.
GB-2202981 and U.S. Pat. No. 5,339,703 describe one known type of identification system which uses a “tree and branch” search methodology to identify transponders. Each transponder has stored therein a unique identity code comprising a plurality of fields, each field holding a respective value selected from a plurality of possible values. Each value is typically represented as a short binary code.
The system identifies transponders by first transmitting a radio frequency initiation signal, in response to which each transponder, within range, will transmit a radio frequency response signal in a time slot corresponding to the value held by the transponder in a first field. For example, a transponder having the value 3 stored in its first field might transmit a response signal in a third time slot, a transponder having the value 1 in a first time slot, etc.
The interrogation device then transmits, in turn, the or each value held by at least one transponder in its first field, in response to which any unidentified transponder, having that value in its first field, will transmit a response signal in a time slot corresponding to the value held by the transponder in a second field.
The interrogation device then transmits, in turn, the or each combination of values held by at least one transponder in its first and second fields, in response to which any unidentified transponder, having that combination of values in its first and second fields, will transmit a response signal in a time slot corresponding to the value held by the transponder in a third field, and so on until the identity of each transponder has been determined.
However, the above process can be time consuming, particularly where the identity code of each transponder comprises a large number of fields.
We have found that, for a given number of transponders, each arranged to reply in one or other of a given number of possible time slots, the number of time slots in which a response is received by the interrogation device may be used as an estimate of the likelihood that two or more transponders have responded in the same time slot.
Based upon this discovery, we have now devised a transponder identification system which is considerably more efficient than existing identification systems.